Introduction to Rebuilding GM/Muncie 4-Speed Transmissions

The Muncie 4-speed was GM’s answer to the T10, and by mid 1962 full working prototypes had been built. It replaced the T10 in General Motors’ cars, starting in February of 1963. Corvettes saw installation by May through June of 1963. It was based on the T10 design, and I have been told that BorgWarner engineers helped design it. In fact, first- and second-speed gears were identical in tooth count and pitch. Some of the later versions of the T10 had 9310 Nickel alloy gears. Because the firstspeed gears were identical to the Muncie except for internal bore and synchro cone sizes, a company called Stahl made an adapter sleeve to be able to use the stronger T10 first gear in the Muncie. The first Muncie in 1963 had a “thin fin” tailhousing and a 6207-style bearing in the front. First gear rode directly on the mainshaft. By 1964, first gear rode on a sleeved bushing, the front bearing diameter was increased, and the webbing on the tailhousing was increased.

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As power levels climbed in 1965 and GM started putting big-block engines in cars, the countershaft diameter increased from 7/8 to 1 inch by 1966. The Muncie had several ratios, case configurations, and spline counts. The last year GM offered the Muncie was 1974. However, some early 1975 cars built in 1974 had Muncies. The Super T10, an improved version of the T10, was reintroduced in 1975 GM cars. Power levels dropped in 1975, requiring different gearing. Since the ST10 had better ratio options, it probably was cheaper for GM to purchase complete transmissions rather than tool up to make different gears for the Muncie.

How to Identify a Muncie

People commonly call a Muncie an M21. Or they think a Muncie passenger- car transmission is the same as a Muncie truck transmission. They are not. What do M20, M21, and M22 actually mean? The “M” codes are RPO (Regular Production Option) codes. They don’t mean Muncie. You can have an M21 4-speed in a 1979 Camaro, and it is actually a closeratio Super T10.

To correctly identify a Muncie, you need three things: First, you need a main case casting number. Second, you need a spline count of both input and output shafts and confirmation of any rings or grooves existing around the input shaft splines. Third, you need the date codes and VIN numbers that help confirm that the case and gears belong together. Tooth counts on the input shaft help confirm a certain gear ratio. However, you may not have access to this information if the transmission is still in the car, or a vendor at a swap meet is unwilling to remove the cover. Sometimes replacement inputs have no ID grooves.

All 26-spline inputs came with 32-spline output shafts and all 10-spline inputs came with 27-spline output shafts. A common mistake is thinking that all “fine spline” 26-spline input shafts are M22 heavy-duty types. This is not true. An M22 gearbox has about a 20-degree helix angle on the gearset, rather than a 37-degree angle.

Also, M22 gearsets were made of a different heat-treated alloy. The straighter angle was designed to produce less end loading of the geartrain noise, thus the nickname “Rockcrusher.” The heat-treated/shotpeen process allowed for more impact of the gears. If the Muncie has a drain plug, it does not necessarily identify it as an M22. Again, the first M22 boxes had drain plugs, but all 3925661 castings had drain plugs. And by 1969, most Muncies had drain plugs.

Serial and VIN Numbers

Serial numbers for Muncie 4-speeds always begin with the letter “P,” which stands for Muncie plant. (Because the Muncie 3-speed used the M-designator, the letter P was used to identify 4-speed transmissions assembled in the Muncie plant). P does not mean passenger car. If P stood for passenger car, you’d see it stamped on every 4-speed, including a T10, but you don’t.

The serial number tells you the date the transmission was built in a particular production year. Serial numbers from 1963 to 1966 included only the month and day. For example, P0102 indicates that the Muncie was built on January 2. From 1967 to1968, the serial number began with a number designator for the year, and then a letter designator for the month. For example, P8A01 means January 1, 1968. One important point: A Muncie dated with a December build date was actually built the prior year. An example would be P8T13—8 is for a 1968 production car, “T” stands for December, and 13 is the day. It was actually built in December of 1967 for the 1968 model year. To confirm this, simply look at the VIN number; it usually has a lower production sequence, justifying an early build.

The 1969 to 1974 Muncies got a ratio designator added to the end of the serial number. An example would be P4D23B. This equates to April 23, 1974, M21 ratio.

Putting It All Together

Start by looking at the main-case casting number. Using the charts, you can determine the date range that a particular casting was used. Check to see if the input shaft has any rings around the splines and then check the stamped P code to help determine ratio.

VIN numbers can help you confirm year, model, and make as well. Later VINs have assembly plant codes and years. For example, a VIN starting with 19N is a 1969 Chevrolet made in the Norwood plant, while one starting with 29N is a 1969 Pontiac made in the Norwood plant. This could be useful for finding out what car the transmission was originally in. However, I always say GM never designed this system for people to accurately decipher codes 40-plus years later at a swap meet. You can be sure that in those years a great number of transmissions have been assembled out of spare parts and cannibalized. So I always like to pull the cover, look at the gears, and count teeth to verify what I think I have.

This shows the typical VIN and serial number stampings on a Muncie.

The top gear is a 10-spline M22; the bottom gear is a 26-spline M20. Note the two grooves on the input designating the M20 ratio.

These GM gears are both 10-spline M20 inputs; however, only the bottom one has OD grooves.

Some input shafts produced by the aftermarket and General Motors have no identifying rings on them. The rings originally corresponded with rings or grooves on the countergear so that the assembler matched a one- or two-ring input with a one- or two-ring countergear. When manufacturing was stopped, GM stopped making inputs with these marks, probably to save machining operations.

Unfortunately, you cannot always use the OD grooves on input shafts as a sure-fire method of identification.

M22 Rockcrushers and the New Muncies

Alan Colvin, author of Chevrolet by Numbers, and Brian Higgins of SK Speed documented that 7/8-inchdiameter M22 countergears did exist. Since the 7/8-inch bore was pre-1966, it suggests that the M22 transmissions were for 1965 cars. Colvin discovered a Chevrolet Engineering Change Recommendation was issued in December 19, 1964, requiring a change of the M20/M21-style transmission to the M22 because of the Grand Sport Corvette Race Program. Prototypes had the 7/8-inch countergear, but 57 M22s were built using 3851325 cases modified with 1-inch bores and added drain plugs, and they had the casting number completely milled off. It’s hard to imagine that 40-plus years later we are still designing transmissions based on the M22 concept.

I’ve always said the M22 was not a drag-race but a road-race transmission. Yet drag racing was much more popular in the 1960s than road racing. Today, we either have helical gears or straight-cut spur gears. Think of the M22 gears as semi-helical. The low helix angle of M22 gears produces less heat, less side loading, and stronger load distribution across the gear tooth. It more efficiently transmits power than a spur gearset, and it is not as noisy. The whine of the M22 gearset is a characteristic of that low helix angle. Four decades ago, we could run street cars with a 4.56 axle ratio, but today this is rarely done. These came with a 2.20 first-gear ratio, and this “dead” ratio just doesn’t work well today. That is why newer 5-speeds are all the rage.

Here is a 3839606 casting, which is very rare. Sometimes odd castings like this show up that are not necessarily documented in restoration guides, but they do indeed exist!

This is a 1965 casting with the factory numbers milled off.

By 2000, almost every Muncie 4-speed that came into the shop had badly worn gears because the main case was either worn or repaired. George Sollish, president of Auto Gear Company, asked me about building a new Muncie 4-speed case and what improvements needed to be made. Using input from myself and several other volume rebuilders, he designed the Muncie Supercase. The case had thicker mounting ears with improved webbing, used a wider sealed front bearing, and incorporated soft plug sealing on the countershaft. This addressed ear breakage problems and leakage issues. The case casting uses state-ofthe- art techniques that yield lessstress risers combined with a better heat-treated aluminum. Gradually, more and more new components were added. I suggested a nodular iron mid plate, and the Superplate was created. A new side cover followed and then new tailhousings. One of the parameters was to make every part backward compatible, so you could bolt-up any older Muncie parts to the new stuff.

By May 2003, we were building complete new Muncie-style transmissions with all-new components. Specialty gear ratios were created to give the Muncie a modern street approach as well as make it more competitive on the race track. You can now get an M22 with a 2.56 first gear or an extra-close-ratio M22X with a 1.17 third-gear ratio. There are also overdrive conversion gearsets. See the Muncie build up in this chapter a great deal of the recently designed parts are used in the build.

Muncie 4-Speed Rebuild

The transmission for this rebuild had quite a few of the common problems associated with 40-yearold transmissions. This Muncie was of the early 1964–1965 era. Because of the age and condition of this transmission, it became a candidate for some of the newer-style upgrades. Muncies really don’t need to have any measurements taken. If something is worn, replace it. Parts are cheap enough. Countergear endplay should be no more than 0.035 inch. If your endplay is excessive, then you have wear on the countergear thrust surface or maincase. You can compensate for wear by making shims out of shim stock to bring the thrust washers more in spec. I still prefer new parts.

Disassembly

Step 1:Start Disassembly

This transmission looks fine from the outside, but do not take things at face value.

Step 2:Note Damaged Countershaft Hole

A closer look reveals a stretched-out countershaft hole, which was hand peened with a punch to try to make the loose shaft fit tighter. That is not the correct way to repair this problem.

Step 3:Slide Cover Interlock 3 Support Pin

The side cover interlock support pin (between the two shifter shafts) is an early design, which can fall inside the transmission during use. Newer styles have an outside support flange.

Step 4:Punch Out the Lock Pin

Use a punch and a hammer to drive the tapered lock pin for the reverse shifter shaft out from the bottom upward. If the pin is frozen, use some heat.

Step 5:Remove Side Cover (Documentation Required)

Shift the transmission into second gear, remove the seven side cover bolts, and remove the side cover. Don’t worry about the shift fork or shifter shafts coming out of the cover. If you are going to reuse them, make a note as to their positions.

Step 6:Inspect Input Shaft

A close look at the input shaft and mating countergear section, also known as the head set, reveals major breakage.

Step 7:Pry Off Retainer

Remove the four bearing retainer bolts and pry off the retainer. Lots of transmissions have stripped threads in this section or oversized bolts and threads.

Step 8:Inspect Front Bearing (Critical Inspection)

The front bearing “gland” nut is on backward, which is a common mistake. Also note the cracked bearing. Lack of a pilot bushing or severe misalignment of the transmission and engine crankshaft usually causes the outer bearing race to crack.

Step 9:Remove Front Bearing Nut

Always chisel off the old nut and replace them with new nuts that are in the rebuild kits. After 40 years or so, they usually seize in place and are difficult to save anyway.

Step 10:Pry Off Front Bearing

Normally, you can pry on either side of the bearing to remove it. However,badly damaged bearings like this one come out in pieces.

Step 11:Remove Reverse Shifter Shaft

Pull out the reverse shifter shaft to disengage the reverse shift fork from the reverse gear. That’s what removing the tapered pin allows you to do. Remove the six tailhousing bolts and pry off the tailhousing.

Step 12:Remove Reverse Gear Parts

Remove the rear reverse idler gear, thrust washer, and idler shaft. Sometimes the shaft sticks in the tail. Notice that the lower reverse gear is a little rounded off from grinding in reverse.

Step 13:Remove Output Shaft and Upper Gearset

Remove the output shaft and upper gearset from the main case. Sometimes you have to pry between the main case and the mid plate. Don’t worry about things flying apart.

Step 14:Remove Input Shaft

Pick out as many loose parts as possible, and then remove the input shaft. Broken inputs, such as this one, make great clutch-alignment tools.

Step 15:Remove Reverse Idler and Thrust Washer (Important!)

Remove forward or inside reverse idler and thrust washer. If you are reusing any thrust washers, make sure that the washer tangs are still able to prevent it from spinning.

Step 16:Inspect Reverse Idler Gear (Critical Inspection)

Inspect the forward edge of the inside reverse idler. The groove cut into the forward edge is due to the 1-2 slider hitting it. This causes a whirring noise heard only in first gear. Worn forks and slider grooves can aggravate this common design flaw.

The front countershaft bore of the main case reveals a severe elongation. Boring it and installing a bushing is a common fix. This needs to be done in a machine shop on a vertical mill. Since this is a 7/8-inch-diameter countershaft, we can just bore the case to the 1-inch size and use a later-style head set. I’ll scrap this case and use a new Auto Gear Supercase. If the countershaft bore is no longer a press fit and the shaft can slide in or out of the case by hand, the case needs to be repaired or replaced.

Step 18:Update Input Shaft

The newer-style M20 input (left) has 21 teeth on a larger diameter, in contrast to 24 teeth on a smaller diameter (right). This makes for thicker and stronger gear teeth, and as a result, the input shaft can transmit more torque.

Step 19:Install Updated Countergear (Performance Tip)

The M20 countergear (right) is the new design with 25 teeth, in contrast to the early 29-tooth finer pitch (left). It also takes a 1-inch countershaft. This upgrade started in 1966. The three holes in the new gear’s front face are for an anti-backlash plate. Remove these plates on every M20 countergear because they have a tendency to break or loosen.

Step 20:Disassemble Geartrain

Remove the 3-4 synchro snap ring to start disassembling the main geartrain. Remove the 3-4 hub and third gear. Some later transmissions may require this to be pressed off.

Step 21:Disassemble Mainshaft

Here’s a trick way to disassemble the mainshaft without having to pull off the speedo gear: Use a snap-ring pliers to remove the rear snap ring that holds the rear bearing to the mainshaft, and let it rest outside the groove.

Step 22:Press Out Gear Set

Support the assembly under second gear and press the output shaft through the whole gearset in one shot.

Step 23: Slide Off Second Gear

The second gear, the 1-2 synchro assembly, first gear and bushing, mid plate, reverse gear, and speedometer drive gear can slide off the mainshaft.

Step 24: Upgrade Synchro Rings (Performance Tip)

We will upgrade this transmission to later-style synchro rings and hubs. The later style (left) has thicker rings, but a thinner hub, as compared to the early set (right). Early non-shouldered rings tend to crack under heavy abuse.

Step 25:Inspect Mid Plate (Performance Tip)

Close examination of the mid plate shows damage due to a spinning countershaft. The plate has an area to lock the pin from spinning. The plate will be upgraded to a nodular-iron Auto Gear mid plate.

Step 26: Remove Bearing from Mid Plate

With snap-ring pliers, spread the rear bearing retaining ring open, and tap the rear bearing out of the mid plate with a punch and hammer. Most plates have snap-ring grooves that have been widened over the years. The widened grooves cause excess geartrain movement. The new nodular-iron plates fit more precisely and help “girdle” the main case as well.

Step 27:Inspect Second and Third Gears (Critical Inspection)

The secondand thirdspeed gears show fairly worn clutch teeth or engagement teeth. The synchro cones have permanent grooves, which may not allow new synchro rings to bite correctly. These will be replaced with new gears.

Step 28:Inspect Front Retainer (Critical Inspection)

The front bearing retainer is grooved because the nut was put on backward. This causes an air leak, which will cause an oil leak. It’s time for a new retainer.

Step 29: A) Inspect Shift Forks (Critical Inspection)

Both shift forks show wear. When forks are worn as much as this one, shift quality can diminish. Combined with worn gears and sliders, the transmission can fall out of gear.

B)Damaged Parts Pile

Here is the scrap pile from this transmission so far. Hopefully, the transmission you rebuild will not be in such poor condition.

Step 30: Remove Reverse Fork and Parts

Remove the reverse fork, reverse shifter shaft, detent ball, and spring. Be careful not to lose the ball and spring.

Step 31: Take Out Shift Shaft Seals

Use a small flathead screwdriver to remove the shift shaft seals in the side cover and tailhousing.

Step 32: Remove Speedometer Driven Gear and Parts

Step 33: A)Remove Rear Seals

Pry out the rear seals with an old screwdriver. Most seal pullers don’t work because the seals are often corroded in place. You must be careful and work the seal from side to side to remove it. Be careful because you can crack early tailhousings in the area because the castings are thin. Remove the bushing.

B)First-Gear Sleeves

All these gear sleeves have the center tapered in for oil collection. People look at the factory first-gear sleeve (left) and think it is worn when it is not. The rare M22 sleeve with milled flats (center) promotes better lube to first gear. The spiral shape of the oil grooves on the new Auto Gear upgrade (right) forces oil into the center of the gear.

Here is how needle bearings and spacers are laid out in a Muncie. The early solid spacer-tube design (top) has four spacers for the 7/8-inch diameter shaft. The split-tube design (bottom) has six spacers with a 1-inch countershaft. You may run into other combinations, such as a solid 1-inch tube with four spacers. Set the countergear up to have the end spacers slightly below the countergear end, usually 0.010 to 0.020 inch per side. Too much clearance can cause the needles to skew. Spacers running too tightly can jam up the needles.

Assembly

AGE Supercase

We are installing the new Auto Gear Equipment supercase (left), which has thicker and stronger mounting ears. It takes a wider front-sealed bearing and has a sealing plug for the countershaft. This resolves the Muncies’ common problem of leaking from the front.

Step 1: Install the Front Thrust Washer

Start assembly with the case face down, and install the front thrust washer.

Drop the countergear in the case, resting it on the front thrust washer. Slip the rear washer between the gear and the case. This technique prevents pushing the thrust washers out of place if the countergear is sitting sideways. It also allows you to see that the washers are in place. Use enough assembly grease to hold washers in place.

B)Sealed Front Bearings

The newer-style sealed front bearing is on the left. You can see how much wider it is compared to the older style on the right.

Step 3:Install Solid Slingers

Install solid slingers (new style) rather than tanged slingers (old style). Olderstyle slingers tend to have tangs that break off. This also acts as a 0.030-inch spacer. The new case design also corrects sloppy alignment of the input and countershafts.

Step 4: Install Input Shaft (Professional Mechanic Tip)

Most books direct rebuilders to install the countergear first. Instead, I suggest you leave the countergear dropped down and install the input shaft with the bearing. As a result, you won’t drive the front bearing on with a hammer while the geartrain is in place and damage the fourth-gear synchro ring.

Step 5:Install Countershaft

Install the countershaft with the flat side facing this position.

Step 6: Use Old Main Case

Here is a trick to assembly a Muncie: Place it on an old case so that you can work at a comfortable level and still keep the input shaft pointing down.

Step 7: Grind a Taper on Reverse Idler Gear

Try grinding more of a taper to the reverse idler gear, so it never hits the 1-2 slider.

Step 8:Insert Needles in Input Cage

Needles must be held in with grease on the input cage. They are positioned in place on the outside of the cage.

Step 9:Complete Main Case

The main case prep has been completed when the input, needles, synchronizer ring, and idler washer have been installed along with a freshly ground idler gear.

Step 10: Position Synchro Hubs (Important!)

Positioning synchro hubs the wrong way is a common mistake. To position them correctly, both hubs must have their extended bosses facing toward the front.

Step 11: Install 1-2 Slider

The sliders, however, face in opposite directions. The 1-2 slider’s taper faces the rear and the 3-4 sliders taper faces the front. For complete synchro ring assembly instructions, refer to page 26.

Step 12: Press Second Gear and Parts onto Shaft

Place the second gear and its synchro ring onto shaft. Next, place the 1-2 assembly and the first-gear sleeve, and press it into place as a unit. Make sure that they all locate correctly on the spline, and that the strut keys are indexed correctly in the synchro ring.

Step 13: Install First Gear Synchro Ring and Parts

Step 14:Position Speedometer Gear

An easy way to determine correct speedometer gear position is to make sure it is close to or on center in the speedo hole of the tail. Put the tail in place and use a felt-tip marker to indicate the center point on the shaft.

Step 15: Heat and Slide Gear into Position

The gear is heated up until it turns blue/brown. When it reaches the correct color, slide it into the correct position with a pair of pliers. Since the gears are soft, this technique avoids damage to the surface.

Step 16: Install Third Gear and Parts

Install third gear, the third-gear synchro ring, synchro assembly, and snap ring on the front part of the mainshaft.

Step 17:Install Reverse Detent Spring and Ball 17

Once you have installed a new bushing and seal in the tail section, install the reverse detent spring and ball, which are held in place with grease. Hold them down with a pry bar and slide the reverse shifter shaft in place. You can also heat up and bend a small putty knife to make a similar tool.

Step 18: Install Shifter Shaft Seal

Once the shaft is in place, install the shifter shaft seal. Do this last, so you’re not fighting the seal’s resistance against the shifter shaft. This procedure actually reduces possible damage to the seal. Once the seal is over the shaft, seat it in place with a deep socket.

Step 19: Install Reverse Shifter Fork

Place the reverse-shift fork in position with grease. Fasten it with either a nut and washer or bolt, depending on the year transmission on the shift shaft. Rotate the shaft in a forward position. The nut and bolt keep the shaft from popping out.

Step 20:Position 3-4 Slider

Position the 3-4 slider slightly forward to install the mainshaft assembly in the main case.

Step 21: Guide Mainshaft Over Countershaft (Important!)

Hold the 3-4 slider in place and guide the mainshaft over the countergear. This is a hard part, and you have to also make sure the fourth-gear ring indexes correctly with the 3-4 synchro assembly.

As you get over the countergear, pull the 3-4 slider back into position, making sure the mid plate dowel is correctly aligned, then sink it all in place.

Step 22: Install Rear Reverse Idler and Parts

Install the rear-reverse idler, thrust washer, and idler shaft in the main case. Align the shaft so that the roll pin faces in this position. Notice the “skin” coats of sealant.

Step 23: Install Tailhousing Main Case (Professional Mechanic Tip)

Install the tail by catching the reverse fork on the reverse gear. Once caught, tap in the shifter shaft with your palm, mate the tail to the main case, and bolt it up.

Step 24: Install Reverse Shifter Shaft Locking Pin

From the top, drive in the reverse shifter shaft locking pin.

Step 25: Tighten Input 25 Shaft to Main Case

Tighten the front nut with either Channel Locks or a special front nut wrench. You can use a little thread locker on the threads. Remember, these are left-handed threaded nuts. Always use a new nut in a rebuild because this functions as a front seal. You have to lock the transmission in two gears at once to keep the input from spinning.

Step 26: Use Sealant on Retainer Bolts

If you are using factory lock plates or stand-alone bolts, always use sealant on the threads. It acts as a thread locker as well.

Step 27: Install Front Retainer

The front retainer is installed with the lock plate tangs bent. The Auto Gear Supercase uses a soft plug on the front of the countershaft. It takes very little pressure to press it in place.

Step 28: A) Rotate Transmission and Check Operation (Important!)

Now is a good time to stick in an old driveshaft yoke and rotate the transmission to see that everything is working smooth, not binding, and shifting. I always splash a little gear lube on the gears when I do this.

B) Needle Bearings in Side Cover

The new Auto Gear side cover uses needle rollers in the side cover. Most older factory covers are extremely sloppy here. Elongated bores cause leaks and geometry problems, which lead to poor shift feel and quality. The rollers keep the shift shafts supported firmly.

Step 29:Deburr Detent

Deburr all detent combs on new and used units with a Dremel cutoff wheel. Check for worn combs. Also note the position of the detent spring. If positioned in this manner, it can never fall into the transmission.

Step 30: Install Side Cover (Torque Fasteners)

Place both the transmission and side cover in second gear. This allows the 1-2 fork to clear the reverse idler boss in the main case. Once in place, align the dowel in the case and tighten the bolts to 18 ft-lbs.

Step 31: Install Speedometer Fitting

The final step is to install the speed fitting. Older fittings had no seals. The new ones do. Don’t bother fixing a rusted leaking fitting, when a new one costs under $10.

Here is the complete build with new hardware. Unless I’m restoring a transmission that requires certain logos on bolts, new grade-5 or grade-8 hardware is preferable. You don’t have to spend time cleaning the hardware, and you don’t risk bolts breaking.

Written by Paul Cangialosi and Posted with Permission of CarTechBooks

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